Method for determining a friction value

ABSTRACT

The invention is based on a process for determination of a coefficient of friction (μ) of a hydraulically activated coupling ( 4, 5, 6 ) or brake ( 7, 8, 9 ) in a load-shifting transmission ( 1 ). It is proposed that a drive shaft ( 14 ) of the transmission ( 1 ) be driven with a specified torque (T), that an output shaft ( 15 ) of the transmission ( 1 ) is blocked with a closed coupling ( 4, 5, 6 ) or brake ( 7, 8, 9 ) during a calibration run, the activation pressure (p) of coupling ( 4, 5, 6 ) or brake ( 7, 8, 9 ) is reduced in accordance with a specified course over time, the differential rotational speed (Δn) between the input ( 17, 18, 19 ) and the output ( 20 ) of the coupling ( 4, 5, 6 ) or brake ( 7, 8, 9 ) is recorded, the activation pressure (p) is ascertained in which the differential rotation speed (Δn) is greater than zero and a separation coefficient of friction (μ separation ) is computed on the basis of torque (T), activation pressure (p) and a construction-conditioned coupling constant. By ascertaining the coefficient of friction μ, the shifting can be conducted over the lifetime of the transmission ( 1 ) with a constant shift quality.

FIELD OF THE INVENTION

The invention concerns a process for determining a coefficient offriction.

BACKGROUND OF THE INVENTION

A process for controlling an automatic transmission driven by aninternal combustion engine is known from EP 0 770 195 B1 where oneshifting takes place from a first into a second gear ratio in that afirst clutch opens and a second clutch closes. A brake through which thetorque of the drive element can be supported on the transmission housingcan also replace a clutch. An electronic control device controls thepressure distribution of the first and second clutch during the shiftingprocess through electromagnetic valves. It determines a first and asecond type of shifting on the basis of input magnitudes whereby thefirst type of shifting is a traction up shift or a reverse thrust andthe second type of shifting is a reverse in up shift or an up shift intraction. The shifting procedures are conducted in the two types ofshifting such that a filling compensation phase is joined with a rapidfilling phase of the hydraulic actuation elements and a load assumptionand is then followed by a closing phase. In the first type of shifting,a gradient assumption phase with a slipping phase follows the loadassumption phase, and a gradient reduction phase precedes the closingphase. In the second type of shifting, a gradient adjusting phasefollows the filling compensation phase and a gradient degradation phaseoccurs before the load assumption phase.

For a fast, jolt-free shifting, it is important that the point, at whichthe clutch or brake are just still or just already are transmitting aload moment, can be rapidly triggered. This point is dependent upon thecoefficient of friction and the coefficient of friction course of theclutch. It is therefore important for shift quality to know the currentcoefficient of friction of the clutch or brake as accurately as possibleand to use it as a basis for control or regulation. Especially with atorque-based, analytic load shifting process, the torque-pressureconnection must be known for the entire gear shifting.

For multiple transmissions, especially construction machinetransmissions, a great number of oils is available. Since thecoefficient of friction of the clutch or brake changes as a function ofthe type of oil, new oil types must be determined for each transmission.If the manufacturer of construction machines or a workshop pours indifferent oil than has been prescribed, the shifting quality worsens.The coefficient of friction can also alter in the course of time throughwear and tear and aging. If the moment-pressure relation no longer fits,a racing of the turbine of a hydrodynamic converter connected upstreamin series or distortion of a transmission is possible

A calibration procedure for couplings in a transmission is known from EP0 709 602 B1. The couplings serve for selective connection of an inputshaft, an output shaft and a large number of gears with one another inorder to bring about a change in the gear step through a selectiveengagement of the couplings. The process includes the following steps:Restraining the output shaft against rotation, determining a referencerotational speed for the input shaft, application of a hydraulicpressure enlargement step value to a selected coupling, measuring therotational speed of the input shaft at the time following theapplication step, comparison of the input rotational speed at the timewith the reference rotational speed after the measurement step,repeating the steps of application, measurement and comparison until therotational speed of the input shaft at the time is smaller than thereference rotational speed, through which it is shown that the inputshaft is subjected to stress, storage of a value which corresponds tothe hydraulic pressure which is necessary at the beginning of thestressing of the input shaft in the electric control system. The pointof load assumption is indeed determined through the known procedure inthat the allocated activation pressure is defined. But no coefficient offriction is determined, which is important for the entire sequence,especially for the gradient adjustment phase of a torque-based analyticload shifting.

Underlying the invention is the objective of again detecting thecoefficient of friction of clutches or brakes in a load-shiftingtransmission under altered operating conditions with as littleexpenditure as possible.

SUMMARY OF THE INVENTION

According to the invention, a drive shaft of the transmission is drivenat a given torque while a driving shaft of the transmission is blockedin the event of a closed clutch or when the brake is blocked during acalibration run. Moreover, the activation pressure of the coupling(clutch or brake) is reduced in accordance with a specified course oftime. At the same time, the differential rotation speed between inputand output of the clutch or brake is recorded, for example in that thedifferential rotational speed of the drive shaft to the output shaft ismeasured. At the time at which the differential rotational speed isgreater than zero, the activation pressure is recorded, and a separationcoefficient of friction is calculated on the basis of the associatedtorque, the activation pressure and a construction-conditioned clutchconstant.

The coefficient of friction of the coupling lining which is a functionof the differential rotational speed, results from the followingrelationship:

Coefficient of friction μ=activation pressure p*torque T/couplingconstant

where the coupling constant is the product of the coupling frictionarea, the friction area figure and the friction radius of the coupling.It arises through the construction of the respective coupling or brakeand does not change during the lifetime or on the basis of differenttransmission oils.

In addition to the separation coefficient of friction which ischaracteristic for the opening coupling or brake, it is appropriate todetermine the dynamic coefficient of friction as well, which ischaracteristic for the closing coupling or brake. For this, an outputshaft of the transmission is blocked with an opened coupling or brakeduring a calibration run. The coupling or brake is acted upon with aconstant activation pressure following a rapid filling. Moreover, theprogress of the differential rotational speed between the input and theoutput of the coupling or brake, and furthermore the course of torqueover time, are recorded on a drive shaft of the transmission. This timethough, a dynamic coefficient of friction is once again calculated onthe basis of construction-conditioned coupling constants, the activationpressure and the allocated torque which applies for sliding friction aslong as the differential rotational speed is not equal to zero.

With the aid of the coefficients of friction ascertained, which areappropriately deposited in a memory module of a transmission controlunit, the shift quality of the load-shifting transmission can be heldconstant under various operating conditions and with differenttransmission oils over its lifetime.

In order that the process of the invention is constantly available, itis provided in accordance with a configuration of the invention that itis deposited as software in a memory module of a transmission controlunit and runs automatically following fetching. This can take place, forexample, in a parked vehicle with parking brakes applied. Instead of thetransmission control unit, obviously any suitable control unit of avehicle or processing machine can be used. Furthermore, it isappropriate for the ascertained coefficients of friction to be assumedby the transmission control unit automatically.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with referenceto the accompanying drawings in which:

FIGS. 1A and 1B are a transmission plan of a multiple gear,load-shifting transmission with an electronic control unit,

FIG. 2 is a diagram for ascertaining a separation coefficient offriction and

FIG. 3 is a diagram for ascertaining a dynamic coefficient of friction.

DETAILED DESCRIPTION OF THE INVENTION

The represented load-shifting transmission 1 is an automatictransmission with a hydrodynamic converter 2 which is driven by adriving pinion 13 and a drive shaft 14 and which drives the change-speedgear connected in series downstream in planetary manner through a hollowshaft 21. The converter 2 can be bridged over with a bypass transmission3 which is arranged between the drive shaft 14 and hollow shaft 21.

The change-speed gear includes three planetary sets 10, 11, 12, thegears and planetary supports of which can be connected with the hollowshaft 21 and an output shaft 15 or with a transmission housing 31through three clutches 4, 5, 6 and three brakes 7, 8, 9 (collectivelycouplings), that result in several forward and reverse gears.

The transmission 1 has a transmission control unit 16 through which theindividual shifts are conducted as a function of operating and drivingparameters corresponding to shift programs deposited in the memorymodules. The transmission control unit at the same time serves toascertain a coefficient of friction :μ_(separation), :μ_(dynamic). Forthis purpose, it has an input 17 for recording the progress of adifferential rotational speed Δn of one of the couplings 4, 5, 6 or thebrakes 7, 8, 9, an input 18 for recording an activation pressure p ofthe switching elements mentioned, an input 19 for recording a torque T,for example, on a drive shaft 14 and an output 20, on which acoefficient of friction μ runs through the time t corresponding to isissued which is calculated on the basis of the input signals inconnection with a coupling constant which is deposited with the programof invention in a memory module of the transmission control unit 16.

In order to ascertain the separation coefficient of friction:μ_(separation) of a coupling 4, 5, 6 or brake 7, 8, 9, the power trainin which the coupling 4, 5, 6 or brake 7, 8, 9 in question liesconnected and the drive shaft 14 is driven with a specified torque Twhile the output shaft 15, for example, is blocked by a parking brake,which is not represented. The activation pressure p of coupling 4, 5, 6or brake 7, 8, 9 is thereupon reduced according to a specified courseover time. At the same time, the differential rotational speed Anbetween the input and the output of coupling 4, 5, 6 or brake 7, 8, 9 isrecorded. At that time, the separation point, when the differentialrotational speed Δn>zero, the activation pressure p is ascertained, andthe separation coefficient of friction :μ_(separation) is calculatedfrom it with torque T and a construction-conditioned coupling constant.

FIG. 2 shows a characteristic curve 22 over time t, which marks thecourse of activation pressure p which drops incrementally up to theseparation point 25. At the separation point 25, the differentialrotational speed Δn, which is represented by characteristic curve 24, isgreater than zero. The coefficient of friction p computed by thetransmission control unit runs over time t in accordance withcharacteristic curve 23. Its value at the separation point 25 isrecorded as the separation coefficient of friction :μ_(separation) anddeposited in a memory module of the transmission control unit 16.

In order to ascertain the dynamic coefficient of friction of a coupling4, 5, 6 or a brake 7, 8, 9, the coupling 4, 5, 6 or brake 7, 8, 9 isfirst of all opened in the power train of the transmission 1. In FIG. 3,the course of the activation pressure p of the coupling 4, 5, 6 or brake7, 8, 9 to be examined is represented by a characteristic curve 27.First a filling pressure 30 is applied that goes in operation during arapid filling phase. It is then followed by a filling compensation phaseafter which the coupling 4, 5, 6 or brake 7, 8, 9 is subjected to aconstant activation pressure p. At the same time, the differentialrotational speed Δn between the input and the output of the coupling 4,5, 6 or brake 7, 8, 9 or the drive shaft 14 and the blocked output shaft15 to be examined is monitored. As soon as the differential rotationalspeed Δn falls, the coefficient of friction μ is calculated on the basisof the activation pressure p and the current torque T on the drive shaft14 of the transmission control unit 16 and deposited in a memory moduleof the transmission control unit.

FIG. 3 depicts the temporal course of the dynamic coefficient offriction μ_(dynamic) as a characteristic curve 28. The correspondingcharacteristic curves 28 and 29 are represented in dotted and dash-dotlines, which result when the activation pressure p is elevated duringits constant phase corresponding to the specified amounts correspondingto the likewise dotted or dash-dot lines.

Reference Numbers

1 Transmission [power train]

2 Hydrodynamic converter

3 Bypass transmission

4 Coupling

5 Coupling

6 Coupling

7 Brake

8 Brake

9 Brake

10 Planetary set

11 Planetary set

12 Planetary set

13 Driving pinion

14 [Axle] drive shaft

15 [Output] drive shaft

16 Transmission control unit

17 Input

18 Input

19 Input

20 Output

21 Hollow shaft

22 Characteristic curve

23 Characteristic curve

24 Characteristic curve

25 Separation point

26 Initial pressure

27 Characteristic curve

28 Characteristic curve

29 Characteristic curve

30 Filling pressure

31 Transmission housing

μ_(separation) Coefficient of friction

μ_(dynamic) Coefficient of friction

Δn Differential rotational speed

p Activation pressure

T Torque

t Time

What is claimed is:
 1. A process for determining a coefficient offriction (μ) of a hydraulically activated coupling (4, 5, 6 or 7, 8, 9)in a load-shifting transmission (1), comprising: driving a drive shaft(14) of the transmission (1) at a specified torque (T); blocking anoutput shaft (15) of the transmission (1) with a closed coupling (4, 5,and 6 or 7, 8, and 9) during the calibration run; reducing theactivation pressure (p) of coupling (4, 5, 6 or 7, 8, 9) in accordancewith a specified course over time; recording the differential rotationalspeed (Δn) between the input (17, 18, 19) and the output (20) of thecoupling (4, 5, 6 or 7, 8, 9); ascertaining the activation pressure (p)at which the differential rotation speed (Δn) is greater than zero; andcomputing a separation coefficient of friction (μ_(separation)) on thebasis of torque (T), activation pressure (p) and aconstruction-conditioned clutch constant.
 2. The process of determininga coefficient of friction (μ) of a hydraulically activated coupling (4,5, 6 or 7, 8, 9) in a load-shifting transmission (1), comprisingblocking an output shaft (15) of the transmission (1) with an openedcoupling (4, 5, 6 or 7, 8, 9) during a calibration run; acting on thecoupling (4, 5, 6 or 7, 8, 9) with a constant activation pressure (p)after a rapid filling; recording over the course time of thedifferential rotational speed (Δn) between the input (17, 18, 19) andthe output (20) of the coupling (4, 5, 6 or 7, 8, 9); recording over thecourse time of torque (T) on a drive shaft (14) of the transmission (1),calculating a dynamic coefficient of friction (μ_(dynamic)) on the basisof a construction-conditioned clutch constant, the activation pressure(p) and the allocated torque (T).
 3. The process according to claim 1,wherein the coefficient of friction (μ) is deposited as software in amemory module of a transmission control unit (16) and runs automaticallyafter being called in.
 4. The process according to claim 1, wherein theascertained coefficients of friction (μ_(separation), μ_(dynamic)) areautomatically assumed by the transmission control unit (16).